feat(billing): yearly-first plan selector and switch-to-yearly prompt

[lindesvard]

Why

Annual plans retain customers much better than monthly ones, but the plan picker defaults to monthly and there's no nudge for established monthly subscribers to switch.

Stacked on #450 (uses the SDK bump and migration ordering from it).

What

• Plan picker defaults to yearly for new subscribers (existing subs keep their interval); defaultInterval prop for preselection; clearer savings copy ("pay for 10 months, get 12") and /yr vs /mo price suffixes.
• Public pricing (home section + slider) gets a "Pay yearly and get 2 months free" line.
• Organization.subscriptionFirstStartedAt: stable tenure anchor from Polar's subscription.createdAt, set in the webhook (kept while the subscription id is unchanged; re-subscribe restarts tenure). subscriptionStartsAt can't measure tenure — it's overwritten with each period start. Backfill wired into sync-subscriptions.ts.
• Dismissible in-app prompt for org admins on monthly plans with 3+ months tenure: "Switch to yearly — 2 months free" → opens the plan picker preset to yearly. Hidden on the billing page and when a state banner is showing; dismissal in localStorage; op.track on click/dismiss.

Ops checklist

• Run sync-subscriptions.ts once after deploy to backfill subscriptionFirstStartedAt (until then the prompt simply doesn't show)

Tests

Typecheck + full suite green (804).

https://claude.ai/code/session_017resSrRFv7wxsc9ifsALAh

Summary by CodeRabbit

• New Features

  • Added yearly billing promotions highlighting two free months on eligible pricing screens.
  • Added a prompt encouraging eligible monthly subscribers to switch to annual billing.
  • Billing plan selection now supports default billing intervals and clearly displays monthly or yearly pricing.

• Bug Fixes

  • Improved subscription tenure tracking by preserving the original start date for ongoing subscriptions and resetting it for new subscriptions.

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• apps/start/src/components/organization/billing-plan-picker.tsx

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📝 Walkthrough

Walkthrough

The change stores the first start time of each current subscription and adds yearly billing promotion messaging. Organization pages can show a dismissible yearly switch prompt for eligible monthly administrators. Billing plan selection now supports a default interval and displays interval-specific pricing.

Changes

Subscription tenure and yearly billing

Layer / File(s)	Summary
Subscription tenure persistence packages/db/prisma/migrations/..., packages/db/prisma/schema.prisma, packages/payments/scripts/sync-subscriptions.ts, apps/api/src/controllers/webhook.controller.ts	Organizations now store subscriptionFirstStartedAt. Synchronization and webhook updates preserve this value for the current subscription and reset it for a new subscription.
Yearly billing selection apps/start/src/components/organization/billing-plan-picker.tsx, apps/start/src/modals/select-billing-plan.tsx	Billing selection accepts defaultInterval, defaults to yearly when no interval exists, labels yearly payment as 10 months for 12 months, and displays /yr or /mo.
Yearly switch prompt integration apps/start/src/components/organization/yearly-switch-prompt.tsx, apps/start/src/routes/_app.$organizationId.tsx, apps/public/src/app/(home)/_sections/pricing.tsx, apps/public/src/components/pricing-slider.tsx	Eligible monthly administrators can open or dismiss a yearly billing prompt. The route controls prompt visibility, and public pricing sections display the two-month yearly-payment promotion.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to d07f0

The yearly plan picker can retain a monthly selection and show a cancellation action before a yearly plan is chosen, which could cause an admin to cancel the existing subscription instead of switching intervals. The stale selection must be cleared or the action suppressed before merge.

Sequence Diagram(s)

sequenceDiagram
  participant OrganizationPage
  participant YearlySwitchPrompt
  participant SelectBillingPlan
  participant BillingPlanPicker
  OrganizationPage->>YearlySwitchPrompt: Render for eligible monthly organization
  YearlySwitchPrompt->>SelectBillingPlan: Open yearly billing plan selection
  SelectBillingPlan->>BillingPlanPicker: Pass defaultInterval="year"
  BillingPlanPicker-->>SelectBillingPlan: Display yearly prices and payment terms

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 12.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 8 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main billing changes: yearly-first plan selection and the switch-to-yearly prompt.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Commit unit tests in branch churn/yearly-first

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[lindesvard]

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Actionable comments posted: 1

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Inline comments:
In `@apps/start/src/components/organization/billing-plan-picker.tsx`:
- Around line 46-50: Update the billing-plan picker’s selected-product
initialization and interval-change flow around recurringInterval and
selectedProductId so a preselected interval differing from the active
subscription cannot retain the old product ID. Only keep or initialize a product
when its interval matches recurringInterval; otherwise clear selectedProductId,
and ensure renderAction does not show the cancellation action when no product in
the selected interval is selected.

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📥 Commits

Reviewing files that changed from the base of the PR and between 30f4b3d and 7a8997b.

📒 Files selected for processing (10)

• apps/api/src/controllers/webhook.controller.ts
• apps/public/src/app/(home)/_sections/pricing.tsx
• apps/public/src/components/pricing-slider.tsx
• apps/start/src/components/organization/billing-plan-picker.tsx
• apps/start/src/components/organization/yearly-switch-prompt.tsx
• apps/start/src/modals/select-billing-plan.tsx
• apps/start/src/routes/_app.$organizationId.tsx
• packages/db/prisma/migrations/20260822100000_subscription_first_started_at/migration.sql
• packages/db/prisma/schema.prisma
• packages/payments/scripts/sync-subscriptions.ts

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[lindesvard]

Consolidated into #455 for easier testing — same commits, all review feedback from this PR already addressed there.